Bravery may not be enough

British politicians, so divided over Iraq, almost all agree that this country's military presence in Afghanistan is necessary. The deaths on Sunday of three British soldiers, taking the total to 100, will not shake their nerve. Gordon Brown, David Cameron and Nick Clegg will each pay tribute to the men in the House of Commons tomorrow and mean what they say: but the policy will not change. Afghanistan, they argue, is a war that Britain cannot afford to lose. Retreat would spell every kind of disaster. Soldiers serving in the heat and dust of Helmand and Kandahar provinces are fighting for what the defence secretary yesterday called, "the noble cause of the 21st century".

Such language is unhelpful. Troops fighting with great commitment and bravery in extreme conditions would be better served if politicians were more modest in the objectives they set and clear about the possibility - and consequences - of failure. It has never been agreed whether Britain is in Afghanistan primarily to protect British interests, fighting terrorism and opium cultivation, or to assist in the reconstruction of Afghanistan; or what would happen if, as looks likely, the mission falls short on all these tasks. Success, of some form, may still be possible, if western commitment lasts. But the confident language still used here has little to do with the reality of the country, seven years after the fall of the Taliban.

Much has changed in Afghanistan since 2001. Even in Helmand, where British troops have spent two years engaged in a conflict much more dangerous than was predicted, things are shifting. The Taliban has moved from direct confrontation to a deadly form of guerilla warfare, attacking troops with improvised explosive devices and suicide bombs, as on Sunday. So the war has changed, but change is not the same as the success claimed by Des Browne yesterday, when he talked of "transforming its heartland from an area of lawless oppression and terrorism to a place of democracy and development".

The reality is a country where security is getting worse and advances - such as democracy, the return of refugees and universal education - are under threat. Suicide bombings are up 600% since 2005, insurgent attacks 400% and even parts of the country once considered safe, such as Herat, are now centres of kidnapping. Holding next year's presidential elections looks an impossibility. Even if Hamid Karzai stands and wins, he is no longer trusted. The international aid and military effort has been caught in a web of confusion and contradiction. Paddy Ashdown, who was supposed to bring some order to it, was vetoed by President Karzai.

None of this makes the British commitment to the country pointless. Rather, this is the moment for the government to narrow and clarify its ambitions. At best the military can create limited stability, creating a space for Afghan capabilities to grow. Last December, the prime minister set out a plan to hand responsibility for security to Afghans. This will mean nothing, though, while the Afghan government is failing. There is justified frustration inside the British military at President Karzai. Britain has spent more than £500m on aid to Afghanistan, on top of military costs of over £1bn a year. But development work is stalling and relations with Kabul are getting worse.

Afghanistan would collapse without western military support; but at some point - not yet - this must end. British troops are doing extraordinary work but without a plan for effective governance and development, they remain caught in a trap, fending off the Taliban, but never overcoming it or able to hand the job to others. President Karzai is asking for £25bn at this Thursday's conference in Paris. This is the moment to confront him with the possibility of defeat and the need for an Afghan response, before Taliban bombers destroy the gains of the last eight years.